Autoamtic gain control circuit



s. l. TOURSHQU Fiied Feb. 24, 1945 UTOMTIC GAIN CONTROL CIRCUIT Nov. 23, 1948.

NRW

Patented Nov. 23, 1948 UNITED` STATES PATENT FFICE Simeon I. Tourshou, Philadelphia, Pa., assigner to Radio Corporation of America, a corporation of Delaware Application February 24, 1945, Serial No. 579,587

6 Claims.

circuits, particularly as applied to gated radio pulse receivers.

The invention will be described with reference to a gated pulse-echo radar system that is designed to4 actuate a warning device if an object moves into a certain region or area. A radar system of this kind is described and claimed in copending application Serial No. 515,496, led December 24, 1943, jointly by Simeon I. Tourshou and Robert Trachtenberg, and entitled Radio pulse echo system, now Patent 2,432,180, issued December 9, 1947. The said region is determined by the timing and duration of a gating pulse that opens up a normally blocked receiver. In order that the warning device shall be actuated only by reflected signal pulses and not by noise pulses, it is desirable to hold the noise pulses at a predetermined amplitude level at the receiver output circuit whereby only the higher amplitude signal pulses will pass through a clipping tube to a relay that operates the warning device.

It has been found that a conventional type automatic gain control circuit does not receive suiicient energy from the noise for satisfactory operation when the duration of the gating pulse is short compared with the signal pulse repetition rate as is the case in the warning system being described.

An object of the invention, therefore, is to provide an improved automatic gain control -circuit for a radar receiver that is rendered eiiective to pass signal periodically and only a small percentage of the total time.

Another object of the invention is to provide an improved automatic gain control system for a radio receiver,

Still another object of the invention lis to provide an improved automatic gain control circuit for a gated radar receiver.

In accordance with the present invention the output -of th'e video `frequency amplier of the receiver is applied to two parallel tuned circuits which are connected in series with each other. One of the tuned circuits is tuned to resonate at a comparatively low frequency determined by the width or duration of the gating pulse so that this tuned circuit integrates th'e noise pulses. The integrated signal thus obtained is applied to the diodel of the automatic gain control circuit.

The other tuned circuit is tuned to a higher frequency determined by the Width or duration ofy the signal pulse.

The signal appearing th'ereacross is` applied through a clipping tube to a relay and Warning indicator circuit. Since the noise level is held -substantially constant by the gain control circuit, the clipping level of the clipping tube can be set so that substantially only the signal pulses are passed on to the Warning circuit relay, thus avoiding false operation.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure 1 is a block and circuit diagram of a radar system embodying the invention and Figure 2 is a group of graphs that are referred to in explaining the operation of the circuit shown in Fig. 1.

Figure 1 shows a pulse-echo radar system comprising a radio pulse transmitter lll, a gate pulse forming circuit H and a timing oscillator I2 that controls the timing of the transmitted radio pulse and the gate pulse.

The receiver of the system is of th'e superheterodyne type and includes a first detector and tuning oscillator indicated at I6, an I.F. ampliner I1, a second detector I8, and a video fre quency amplier i9. The output of the amplifier I9 (consisting of a signal pulse.48 and noise as indicated at graph a in Fig, 2) is applied to an amplifier tube '2| that has in its anode circuit a noise integrating tuned circuit 22 that is resonant at a `comparatively low frequency and a signal supplying tuned circuit 23 that is resonant at a higher frequency.

The integrated noise energy from the tuned circuit 22 (indicated at graph b in Fig. 2) is applied through a coupling capacitor 24 to a diode 26 comprising part of the automatic gain control circuit which will be described more fully hereinafter.

The signal pulse 48 and the accompanying noise (indicated at graph c in Fig. 2) from the tuned circuit 23 are applied to a vacuum tube 27 that is biased beyond cut-oiT suiciently to pass only the signal pulses 48 as indicated by th'e vdotted line 20 at graph c of Fig. 2. The signal pulses 48 passed by the tube 2l are supplied to a suitable relay, such as a Thyratron, for operating a Warning indicator such' as a light or a bell as indicated at 2B. The anode voltage for the tube 27 may be applied through a resonant circuit 2S tuned to the same frequency as the tuned circuit 23.

Referring more specifically to the I.F. ampliner Il, which is conventional in design, it may consist of several stages similar to the one illustrated which' includes a tuned coupling circuit 3! and a pentode 32. The coupling circuit 3l comprises a tunable inductance coil 33, shunted by a damping -resistor 34, a tunable inductance coil 36, and a coupling capacitor 31.

The pentode 32 is provided with a negative operating bias for the control grid by means of a cathode resistor 38 and a shunt capacitor 39. Gain control bias is applied to the control grid 4l of pentode 32 from the automatic gain -contrci circuit through a filter resistor 42 and th'e coupling coil 36. A filter capacitor 43 is connected between ground and the junction point of resistor i2 and coil 36.

A positive gate pulse from the circuit il is applied to the screen grid M =of the pentode 32 through a filter resistor 46. A filter capacitor is shown at fi?. Since the only voltage applied to the screen grid 44 is the gate pulse voltage, the pentode 32 will pass signal only While the gate pulse is on the screen grid. This feature is claimed in the above-identified r'ourshou and Trachtenberg application. If a reilected signal pulse is received during this gating period, it will appear in the output of the video frequency amplifier I9 as indicated at d8 in graph c of Fig. 2.

Referring more in detail to the automatic gain control and associated circuit, the last video frequency amplifier stage 2| is resistance-capacity coupled to the preceding stage and is provided with a grid current limiting resistor l The tuned circuit 22 preferably is tuned so that its resonant period is equal to twice the duration of the gate pulse whereby the applied energy (graph a of Fig. 2) is integrated to produce the voltage Wave 52 shown in graph b of Fig. 2. The wave 52 is rectified by the diode 2B thus placing a direct-current charge on the capacitor 2d which charge leaks off slowly through a leak resistor '53. As a result, the upper end of resistor 53 has a negative voltage with respect to ground that increases in amplitude with respect to ground in response to any increase in the amplitude of the noise signal. This negative voltage is applied as a gain control voltage over a conductor 5d to the control grid 4i of the I.F. amplifier tube 32 whereby the noise level is held substantially constant at the output of the amplifier tube 2|. It may be noted that the signal pulse 48 will have practically no effect on the gain control because it is of short duration compared With the gate pulse duration and, therefore, has small energy content as compared with that of the noise passed during the gate period.

It may be noted that the ration of gate pulse duration to signal pulse duration preferably should be about l0 or l5 to 1 or more for best operation of the invention.

The video frequency output signal shown at graph a. (Fig. 2) also feeds through the capacitor of the tuned circuit 22 to energize the tuned circuit 23 which preferably is tuned to have a resonant period of a duration twice the duration of the signal pulse 48. Therefore, the signal pulse 43 appears across the tuned circuit 23 as a damped sine wave as shown in graph c (Fig. 2), and the positive half cycle 48 is passed through the clipping tube 27 to the exclusion of the noise whereby false operation of the warning indicator is avoided. It should be understood that the clipping tube 2l may be adjusted to function as an amplifier tube without any clipping action if the relay and indicator circuit 28 is adjusted to respond only to signal that exceeds the noise voltage level.

I claim as my invention:

l. In a radio receiver wherein the receiver output energy comprises a received signal and accompanying noise, and wherein the duration ofr the received signal is short compared with that of the noise, means for receiving and demodulating the received signal to obtain a video frequency signal, said receiving means including an amplifici', a pair of parallel resonant circuits which are connected in series with each other and which are tuned to resonate at compartively low and comparatively high frequencies, respectively, means for applying said video frequency signal and the accompanying noise signal across said resonant circuits in series whereby integrated noise voltage appears across the circuit tuned to the low frequency and whereby the received signal accompanied by noise appears across the circuit tuned to the high frequency, means for controlling the gain of said amplifier in accordance with the amplitude of said integrated noise voltage to hold the noise output substantially constant, and means for applying the signa1 appearing across the high frequency tuned circuit to a utilization device.

2. YIn a radio pulse receiver wherein the receiver is operative to pass signals only during periodic gating periods which are a small percentage of the total time, means for receiving and demodulating reflected pulses accompanied by noise to obtain a video frequency signal, said receiving means including an amlplier, a pair of parallel resonant circuits which are connected in series with each other and which are tuned to resonate at comparatively low and comparatively high frequencies, respectively, means for applying said video frequency signal across said resonant circuits in series whereby integrated noise voltage appears across the circuit tuned to the low frequency and whereby the signal pulse accompanied by noise appears across the circuit tuned to the high frequency, means for controlling the gain of said amplifier in accordance with the amplitude of said integrated noise voltage to hold the noise output substantially constant, a pulse-reception indicator, and means for applying the signal appearing across the high frequency tuned lcircuit to said indicator.

3. In a radio pulse receiver wherein the receiver is operative to pass signals only during periodic gating periods which are -a small percentage of the total time, means for receiving and demodulating reflected pulses laccompanied by noise lto `obtain a video frequency signal, said receiving means including an amplifier, `a pair of parallel resonant circuits which are connected in series with each other and which are tuned to resonate at com.- paratively low and comparatively high frequencies, respectively, means for applying said video frequency signal across said resonant circuits in series whereby integrated noise voltage appears across the circuit tuned to the low frequency and whereby the signal pulse accompanied by noise aP- pears across the circuit tuned to the high frequency, means for rectifying said integrated noise voltage thereby producing .an automatic gain control Voltage, means for applying said gain contr-o1 voltage to said amplifier in negative feed-back relation to hold the noise output level substantially constant, a pulse-reception indicator, and means for .applying :the signal pulse appearing across the high frequency tuned circuit to said indicator.

4f. In a radio pulse receiver wherein the receiver is operative to pass signals only during periodic gating periods which are a small percentage of the total time, means for receiving :and demodulating reflected pulses accompanied by noise to obtain a video frequency signal, said receiving means including an amplifier, -a pair of parallel resonant circuits which are connected in series with each other and which are tuned to resonate at cornparatively low and comparatively high frequencies, respectively, means for applying said video frequency signal across said resonantl circuits in series whereby integrated noise voltage appearsv across the circuit tuned to the l-ow frequency and whereby the signal pulse accompanied by noise appears across the circuit tuned to the high frequency, means for rectifying said integrated noise voltage and thereby producing an automatic gain control voltage, means for applying said gain control voltage Ito said amplifier in negative feed-back relation to hold the noise output level substantially constant, a clipping circuit and a pulse-reception indicator, and means for applying the signal appearing across the high frequency ltuned circuit to said indicator through said clipping circuit, the clipping level ofsaid clipping circuit being set at a level to pass Va received signal pulse substantially to the exclusion of the accompanying noise.

5. Ina radio pulse receiver wherein the receiver is operative to pass `signals only during periodic gating periods which are ra small percentage of the total time, means for receiving and deniodulating reiiected pulses accompanied by noise to obtain a video frequency signal, said receiving means including an amplifier, a pair of parallel resonant circuits which are connected in series with each other and which 4are tuned to resonate at comparatively low and comparatively high frequencies, respectively, said low frequency having a period substantially equal to twice the duration of said gating period and said high frequency having a period substantially equal 'to twice the duration of said signal pulse, means for applying said video frequency signal across said resonant circuits in series whereby integrated noise voltage appears across the circuit tuned to the low frequency and Iwhereby the signal pulse accompanied lby noise appears across the circuit tuned lto the high frequency, means for deriving :an automatic gain control voltage from said integrated noise voltage, and means for :applying said gain control voltage Ito said amplifier in negative feed- 4*' back relation .to hold the noise 4output level substantially constant.

6.` In a pulse-'echo receiver wherein the receiver is operative to pass signals only during `periodic gating periods which are a small percentage of the 25 circuit to said indicator.

circuits which are connected in series with each other and which are tuned to resonate at comparatively low and comparatively high frequencies, respectively, said low frequency having a period substantially equal to twice the duration of 'l0 said gating period and said high frequency having a period substantially equal to twice lthe duration of said signal pulse, means for applying said video frequency signal across said resonant circuits in series whereby integrated' noise voltage appears across the circuit tuned to the low frequency and whereby the signal pulse accompanied by noise appears across the circuit tuned to .the high frequency, means for deriving an automatic gain control voltage from said integrated noise voltage,

means for applying said gain control vol-tage to said amplifier in negative feed-back relation to hold the noise output level substantially constant, a warning indicator, and means for applying the signal appearing across the high frequency tuned SIMEON I. TOURSHOU.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATESI PATENTS Number Name Date 1,924,174 Wolf Aug. 29, 1933 2,153,202 Nichols Apr. 4, 1939 2,157,677 Runge May 9, 1939 2,181,309 Andrieu Nov. 28, 1939 2,188,658 Hershey Jan. 30, 1940 2,223,995 Kotowski et al Dec. 3, 1940 2,329,570 Wellenstein et al. Sept. 14, 1943 2,371,988 Granqvist Mar. 20, 1945 2,414,992 Wheeler Jan. 28, 1947 2,429,513 Hansen Oct. 21, 1947 FOREIGN PATENTS Number Country Date 480,572 Great Brit-ain Feb. 24, 1938 552,072 Great Britain Mar. 22, 1943 520,060 Great Britain Apr. 29, 1940 108,556 Australia Sept. 14, 1939 

